the limits of cd metrology - national institute of standards

March 15, 2005 R

The Limits of CD Metrology

Intel CorporationBryan J. Rice, Heidi Cao, Michael Grumski, Jeanette Roberts

Various Metrology SuppliersLawrence Berkeley National Laboratory

March 15, 2005 R

Outline

CD DefinitionITRS CD Metro RequirementsCXRO Wafers with 32nm+ Node FeaturesCD SEM ResultsScatterometry ResultsOther Technology ResultsSummary and Conclusions

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CD Metrology in the 32 nm node (and beyond)?

The good old days: 0.5 micron lines and holes

Today: 50 nm 20 nm 15 nm 10 nm

25 nm

15nm

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CD Definition for in-fab CD Metrology

The present paper is concerned exclusively with characterizing in-fab CD measurement technologies.What do these technologies need to measure? Some combination of technologies must be able to measure all of these:– Classic CD (i.e. width), quantitative statistics (mean, sigma, etc)– LWR (Line Width Roughness)– Profile (sidewall angle for simple cases, curvatures for complex cases)– High aspect ratio features (>10:1)

And must be– Non-destructive (i.e. measured part must still operate normally)– High throughput (for process control and scanner qual applications)– Highly repeatable and reliable

Over the past few years Intel has evaluated CD SEM, scatterometry, atomic force microscopy, dual incident beam, and HV SEM technologies and has supported experiments with CD-SAXSSo why has Intel bothered?

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CD Metrology ITRS RoadmapThe following values are from the International Technology Roadmap for Semiconductors, 2004 Update, Lithography and Metrology sections, with the exception of the rows marked with *

Manufacturable solutions known; Manufacturable solutions not known

Year (ITRS) 2007 2010 2013 2016*Year (2 Year Roadmap) Today 2007 2009 2011*Year Tools Needed for Dev. 2003 2005 2007 2009*Year Tools Needed for Res. 2001 2003 2005 2007Technology Node 65 nm 45 nm 32 nm 22 nm1/2 Pitch (nm) 65 45 32 22Contact in resist (nm) 80 55 40 30Contact post etch (nm) 70 50 30 21Aspect ratio 15:1 15:1 20:1 20:1Gate in resist (nm) 35 25 18 13Gate post etch (nm) 25 18 13 9Gate CD control 3σ (nm) 2.2 1.6 1.2 0.8Metro CD 3σ precision (P/T=0.2) 0.45 0.32 0.23 0.16

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CXRO Wafers

Intel funded the Center for X-ray Optics at LBNL to create an e-beam writing process capable of producing 32nm node features.4” wafers were patterned by the Nanowriter, a 100keV ebeam writer, to produce nested lines, isolated lines, and contact holes as small as:

Both resist and etched substrate wafers were fabricated. Silicon was used for the etched line/space wafers while oxide (HSQ) was used for the etched contact wafers.Etch processes were developed specifically to created the etchedsubstrates imaged in this presentation.These wafers were used to evaluate the CD SEM and scatterometry technologies and have been supplied for CD-SAXS experimentation

Size (nm) PitchNested Lines 36

1645

1:1, 1:3Iso Line 1:10Con 1:1, 1:2

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CXRO Wafer Process Flow

Line/Space Wafers

Silicon (4” wafer)

110 nm HSQ

Silicon (4” wafer)110 nm Si

Resist Lines (HSQ used at ebeam-resist)

Etched Silicon Lines/Spaces

Contact hole Wafers

110 nm ZEP

Resist Lines (ZEP ebeam-resist)

Etched contacts

Silicon (4” wafer)100 nm HSQ

Silicon (4” wafer)100 nm HSQ

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CD SEM Results – Isolated Resist Lines

Lines: Nominally 16 nm on a 176 nm pitch.Static Repeatability achieved today:

– ~ 0.2 nm 3σ

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CD SEM Results – Resist contact holes

Contact Holes: Nominally 45 nm on a 1:1 pitch.Static Repeatability achieved today:

– ~ 0.4 nm 3σ

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CD SEM Results – Etched Resist Lines

Lines: Nominally 16 nm on a 176 nm pitch.Static Repeatability achieved today:

– ~ 0.2 nm 3σ

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CD SEM Results – Etched contact holes

Contact Holes: Nominally 45 nm on a 1:1 pitch. Static Repeatability achieved today:

– ~0.4 nm 3σ

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CD SEM Results

Demonstrated clear ability to resolve both edges on smallest isolated linesCD SEM remains the technique of choice for LWR measurementsMeasurements demonstrated that damage will continue to be an important concern for CD SEM technologiesKey Conclusion: CD SEM technology is capable of imaging features at the 32 nm technology node, but the tools must undergo continuous improvement to be ready for HVM in the 32 nm node.

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Scatterometry results – CD Measurements Resist Bottom CD

0.010.020.030.0

40.050.060.0

0 10 20 30 40 50

Drawn CD (nm)

OC

D R

esis

t BC

D (n

m

Supplier A

Supplier B

Line CD’s: Multiple suppliers obtained good solutions for the smallest lines (1:10) in patterned resist and etched silicon.For resist lines, suppliers accurately predicted straight sidewall profiles.For etched silicon lines, all suppliers found poor sidewall angle sensitivity (likely due to small sample volume).

Etched Si Bottom CD

0.010.020.030.0

40.050.060.0

0 10 20 30 40 50

Drawn CD (nm)

OC

D E

tche

d Si

BC

D (n

m

Supplier X

Supplier Y

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Scatterometry results – CD SEM Comparison

Line CD’s: In general, scatterometry data (bottom CD shown) correlated extremely well with CD SEM data, even down to 20 nm.

CD vs. OCD Correlation

R2 = 0.9704

20

30

40

50

20.0 30.0 40.0 50.0

OCD (nm)

CD S

EM (n

m)

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Scatterometry results – Sematech results

Ben Bunday et al, SPIE 2005 paper (in press).

Ben Bunday presented results at SPIE 2005 of a comprehensive Sematech study of scatterometry tools. His data also suggest scatterometry correlates well with actual CD’s, although his data indicate poor OCD accuracy.

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Scatterometry results – CD Sensitivity

Scatterometry exhibits some sensitivity to small CD differences even at CD’s < 20 nm. (Note: curves below are fits to actual CD data)

α & β Sensitivity to CD

-1-0.8-0.6-0.4-0.2

00.20.40.60.8

Wavelength

α &

β

α, 19 nmα, 21 nmα, 23 nmα, 25 nmβ, 19 nmβ, 21 nmβ, 23 nmβ, 25 nm

Smaller CD,

Decr. sens.

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Scatterometry results – SWA Sensitivity Scatterometry exhibits less sensitivity to sidewall angle (SWA) on etched silicon lines with Si height of 30 nm. (Note: curves below are simulations with SWA=68° to 78 °.)

α/β Sensitivity to Sidewall Angle

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

Wavelength

α/β

Min SWA234567891011Max SWA

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Slide courtesy of Ben Bunday, SPIE 2005, in press.

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Scatterometry Results

Key Result: Scatterometry demonstrated the ability to resolve 32 nm node CD’s.Resist profiles were modeled accurately (choice of SWA=90° is corroborated by cross section images). – Feature heights for resist generally near 55-100 nm.

Less success in modeling etched silicon features. Feature height for etched silicon lines was about 30 nm. Feature height WAS accurately modeled, but SWA was generally not accurately modeled.– Feature quality was worse for the etched lines than for the resist

lines and accounts for some reduction in measurement quality.Key Conclusion: CD sensitivity to small CD’s and SWA sensitivity for thin features must be improved to meet 32 nm HVM targets. If sensitivity solutions are found scatterometry will be the profile measurement standard.

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AFM

The capability of atomic force microscopes is primarily dependent upon the tip technology and the control mode.Traditional (C and Si) tip sizes of about 50 nm are currently available; carbon nanotube tips (CNT’s) as small as 20 nm have been reported1.Today, in order to measure non-reentrant profiles it is possible to use straight, sharp tips like CNT’s. Using a control mode like that proposed in Ref [1], the “Step In” mode, it should be possible to measure 32 nm node features.

1Morimoto et al, Proc SPIE 5038 (2003), pp 636

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AFM Results

AFM’s primary difficulty arises when measuring reentrant profiles.This requires using a “boot” shaped tip and the “tapping mode” of operation. Contact forces and resonant frequency add additional space requirements of 20-30nm above the physical tip size limiting space/hole capability to ~80 nm.Key Result: AFM can measure P1268 isolated lines today, and provides unmatched 3D profile capability on non-reentrant features, but is not capable of measuring reentrant features from the 65nm node & beyond.

Trace from failed space measurement

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Dual Beam Results

Key Result: Static and Dynamic precisions lag CD SEM precisions.Problems: Difficulty obtaining high fidelity cross-sections of resist features. Ga contamination for front end processes is an issue.Advantages: 2-8 minutes per high quality cross sectional image; capable of utilizing full 12” wafers rather than coupons; in situ decoration techniques.Key Result: Dual beam offers promise for low-sample rate inline CD Metro; could replace many current analytical-SEM tasks (and could be in fab). Primary use would be development and inspection.

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HV SEMIt was recently proposed1 that using high energy (50–200 keV) electrons might provide improved imaging compared to the traditional secondary electron (SE) used into today’s CD SEM’sPositives:

– Little energy deposited in the resist (i.e. no line slimming)– Improved resolution compared to SE SEM

Negatives: – Potential for transistor damage

Intel collaborated with Hitachi High Technologies to determine if transistor damage results from the use of 50-200 keV electrons in an HV SEMWe irradiated specific transistors on fully integrated Pentium IV processors fabricated using the 0.18 µm process and performed a variety of electrical tests on these devices.The wealth of data precludes full description here, so I will only show the drain current results

– The green data are the control set (no irradiation)– The red data are the lump distribution of all irradiated data– The vertical axis shows (Post – Pre)/Pre as a %.

1 David Joy, SPIE presentation, 2002.

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HV SEM Results by Treatment: Drain Currents

NM

OS

Dra

in "O

n" C

urre

nt (%

Cha

nge)

-0.5

0

0.5

1

1.5

Control Irradiated

Treatment

Control

Irradiated.01 .05.10 .25 .50 .75 .90.95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

NM

OS

Dra

in "D

rive"

Cur

rent

(% C

hang

e)

-0.5

0

0.5

1

Control Irradiated

Treatment

Control

Irradiated.01 .05.10 .25 .50 .75 .90.95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

PM

OS

Dra

in "O

n" C

urre

nt (%

Cha

nge)

-9-8-7-6-5-4-3-2-101

Control Irradiated

Treatment

Control

Irradiated

.01 .05.10 .25 .50 .75 .90.95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

PM

OS

Dra

in "D

rive"

Cur

rent

(% C

hang

e)

-6

-5

-4

-3

-2

-1

0

1

Control Irradiated

Treatment

Control

Irradiated

.01 .05.10 .25 .50 .75 .90.95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

“On” “Drive”

NM

OS

PMO

S

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HV SEM Results and ConclusionsFor all doses and energies we found that the devices were affected by irradiationThe data show a logic device reliability failure issue. No device exposed to these conditions, even the lowest dose, could be sold.Significant reduction in dose must be achieved before the primary electron-based SEM may be used for CD metrology on product material.Follow-up experiments could determine the maximum allowable dose on logic devices.

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Summary and Conclusions

Evaluation of CD Metro technologies at 32 nm node dimensions shows – CD SEM continues to be capable at 16 nm lines and 45

nm holes– Scatterometry can predict the CD correctly for 16 nm

lines, but profile fidelity is still an open question.– AFM is not capable of measuring 32 nm node features

if they are reentrant.– Dual beam has difficulty imaging resist features and is

locally destructive, but offers promise for characterization.

– HV SEM causes damage to devices and is (in its present form) unsuitable for use as a CD metrology solution. A lower dose version might bear revisiting…

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Future WorkTechnology evaluations with 22 nm node features are planned using EUV lithography and Intel process technologies in 2005-2006 timeframe. CD SEM and scatterometry evals have already begun on available feature sizes.

CD = 50 nm, Pitch = 550 nmCD = 100 nm, Pitch = 200 nm

45nm 1:1 27 nm ~50 nm

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AcknowledgementsThe authors would like to thank the following for their contributions to the present work

– CXRO Lab at LBNL, especially Deirdre Olynik and Alex Liddle– Metrology equipment manufacturers (you know who you are!)– The device experts are Intel in PTD and CR for the images of

their prototype devices– David Joy for his ideas on future metro technologies– Rex Frost and Brian Coombs for help in creating some of the

wafers used in the evaluations– Jose Maiz for his help in understanding reliability failures– Gary Crays for his help in analyzing e-test data– Ben Bunday for allowing me to present some Sematech data

the limits of cd metrology - national institute of standards

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